This invention relates to perfluorocarbon copolymer films and a process for their preparation.
Films of melt fabricate perfluorocarbon copolymers such as those made from tetrafluoroethylene/perfluoro(alkyl vinyl ether), tetrafluoroethylene/hexafluoropropylene and tetrafluoroethylene/hexafluoropropylene/perfluoro(alkyl vinyl ether) copolymers, are well known in the art. These copolymers are melt extruded through an orifice to form a film and then quenched, such films being useful for chemical shielding, release surfaces and specialty glazing. These films, however, suffer the disadvantage of having excessive creep, i.e., deformation under moderate loading over extended time periods, and an extremely low tensile modulus.
Further, perfluorocarbon copolymers have been extruded in tubular form and blown to about twice the diameter of the unblown tubing, thereby increasing the strength of the film and allowing the blown tubing to be heat-shrunk around, e.g., process rolls in order to impart a protective, low friction surface to such rolls. Upon heating, however, such films tend to expand in the direction perpendicular to the direction of stretching effected by blowing and, hence, are not practical for heat shrinking around rectangular surfaces where tension in such perpendicular direction is required to hold the film taut.
Also, U.S. Pat. No. 3,770,711 mentions uniaxially stretching a film of tetrafluoroethylene/hexafluoropropylene copolymer up to about 40 percent, and tetrafluoroethylene/perfluoro-(propyl vinyl ether) copolymer up to 250 percent, in a tensile tester. These films, however, as seen below, also expand in the direction perpendicular to stretching upon heating.